You have more nerve than most trolls, Photozopia. You just keep arguing and insulting more knowledgeable people, even after your arguments have been demolished. You actually think a hood has something to do with exposure? You never heard that extraneous light which is not on a path through the aperture to the sensor might have an effect on IQ? (not "exposure") As RGBaker says, such light causes veiling flare (even when it does not cause the more obvious ghost images in the image), which reduces contrast, saturation, and overall IQ. For the same reason that it is desirable to use a hood in the first place, it is desirable to use one which cuts out as much non-image-forming light. It's the same reason we have multicoating, the same reason the inside of a camera or lens is painted black (or made of black plastic).

You might want to look in one of those "photo textbooks" you talk about so glibly and read about how lens hoods improve IQ. You might also want to go into the studio of a real pro photographer and see a compendium lens hood in use: a rectangular hood which can be extended as far as needed without causing vignetting.